Page:Historic highways of America (Volume 15).djvu/188

 having the same name, such as traps, granites, quartzites, etc., vary almost as much in different localities in their physical road-building properties as they do from rocks of distinct species. This variation is also true of the mineral composition of rocks of the same species, as well as in the size and arrangement of their crystals. It is impossible, therefore, to classify rocks for road-building by simply giving their specific names. It can be said, however, that certain species of rock possess in common some road-building properties. For instance, the trap rocks as a class are hard and tough and usually have binding power, and consequently stand heavy traffic well; and for this reason they are frequently spoken of as the best rocks for road-building. This, however, is not always true, for numerous examples can be